1. Field of the Invention
The present invention relates to an air intake side secondary air supply system for an internal combustion engine, for controlling the amount of the secondary air to be supplied to the engine.
2. Description of Background Information
In an internal combustion engine provided with a three-way catalytic converter in the exhaust system, it is general to control the air/fuel ratio of the engine around a stoichiometric value (14.7:1 for example) in accordance with the operational conditions of the engine in order to provide an optimum operation of the catalytic converter. The so called air intake side secondary air supply system is an example of the air/fuel ratio control system of this type. In the air intake side secondary air supply system, there is provided a secondary air passage leading to a portion of an intake air passage downstream of the throttle valve and the amount of the secondary air passing therethrough is varied to adjust the air/fuel ratio of the mixture to be supplied to the engine.
As an example of the air intake side secondary air supply system, the present applicant has proposed a system in which an air control valve is disposed in the secondary air supply passage, for varying the sectional area of the secondary air supply passage in accordance with the pressure level in a pressure chamber of the air control valve. Further, the actual air/fuel ratio is detected from an oxygen content of the exhaust gas, and a first control pressure is applied to the pressure chamber of the air control valve to gradually increase the sectional area of the secondary air supply passage when the detected air/fuel ratio is rich and a second control pressure is applied to the pressure chamber of the air control valve to gradually decrease the sectional area of the secondary air supply passage when the detected air/fuel ratio is lean.
Thus, in the above described air intake side secondary air supply system proposed by the applicant, the control speed of the enrichment or leaning control of the mixture is determined by the sectional area of the first control pressure passage or the second control pressure passage to the pressure chamber of the air control valve. However, in some states of the engine operation, it is suitable to change the speed of the air/fuel ratio control depending on whether the control is the control to the richer side or the control to the leaner side.
Accordingly, it is desirable to design the air/fuel ratio control system so that the speed of the control to the richer side and the speed of the control to the leaner side can be established independently from each other.
Further, in the case of an internal combustion engine equipped with the three way catalytic converter, the air/fuel ratio in which the efficiency of the catalyst becomes optimum, varies depending on the kind of the noxious component. Thus, it is also desirable that the central value of the air/fuel ratio control can be varied according to the state of the engine operation, so as to improve the efficiency of the emission control system.